Allogeneic bone marrow transplantation (allo-BMT) is a potent form of immunotherapy with a potential for curing many hematological diseases through its graft-versus-leukemia/tumor effect (GVL). However, graft-versus-host disease (GVHD), the most frequent and severe complication of allo-BMT has limited the application of this effective therapy. Several lines of evidence have established a critical role for donor na?ve conventional cells (Tcon) in the induction, and to donor natural T regulatory cells (Tregs) in the regulation of GVHD. Nevertheless, there are significant gaps in our understanding of the molecular mechanisms that dampen Tcon cells and simultaneously enhance Treg functions. Recent data have demonstrated that microRNAs (miRs), a class of small, noncoding RNAs, play a significant role in orchestrating immune responses. The role of these critical molecular regulators in the modulation of allo-T cell responses in GVHD and GVL has heretofore been unexplored. Exciting preliminary data generated by us show that the overall miR landscape, and specifically miR-142, is significantly altered in dendritic cells (DCs) and alloreactive T cells. T better decipher these novel observations, we have generated miR-142 knock-out (KO) mice and observed that miR-142 deficiency in donor T cells mitigated GVHD. In this proposal we will build on our exciting preliminary observations and test the central premise that deficiency of miR-142 will reduce GVH responses through impairment of donor Tcon cells and enhancement of donor Treg functions. Utilizing complementary approaches with miR-142 knock-out (KO) mice and blockade with novel locked nucleic acid (LNA) oligonucleotide we will systematically test the cell autonomous role and function of miR-142 in Tcon and Treg cells after allo-BMT. If successful, our proposal will lead to targeting of miRs, specifically miR-142, as a novel strategy for prevention of GVHD and may also be salutary in a broad array of immune mediated diseases. The specific aims are: Specific Aim (SA) 1: To determine the role of miR-142 expression in donor derived na?ve T conventional cells (Tcon) in the induction of GVH responses after allogeneic BMT. We will test the hypothesis that deficiency of miR-142 in na?ve Tcons will impair their functions and reduce the severity of GVH responses. Specific Aim (SA) 2: To analyze the role of miR-142 in the negative regulation of GVHD by the donor derived natural Foxp3+ T regulatory cells (Tregs) after allogeneic BMT. We will explore the hypothesis that deficiency of miR-142 in donor derived Tregs will enhance their GVHD protective effects. Together, the above studies will illuminate a novel, heretofore unexplored role of miR mediated molecular regulation of GVH responses. If successful, may lead to innovative therapeutic strategies targeting miR-142 to impair Tcon responses or enhance Treg responses. Such an approach may prove to be beneficial in a broad array of many other T cell mediated immune diseases.